1. Field of the Invention
The present invention relates to efficient processing and state management of containers and various equipments in ports, and more particularly, to an active radio frequency identification (RFID) system for port logistics and a communication method in the RFID system for improving the tag recognition rate when active RFID tags having a frequency of 433 MHz are attached to each item and tag information is collected by using a reader.
2. Description of the Related Art
A radio frequency identification (RFID) system mainly used in port logistics is an active RFID communication system based on a frequency of 433 MHz and conforms to the international standard, International Standardization Organization/International Electrotechnical Commission (ISO/IEC) 18000-7. Such a standard defines various command packets of a reader, which can be classified into a broadcast command and a peer-to-peer (P2P) command, and defines the form of response messages of the tag with respect to the commands. In U.S. electronic seal (e-seal) tags have been studied for tracing flowing of imported and exported items and strengthening security of goods. In some ports in U.S and Europe, active tags having a frequency of 433 MHz are attached to containers and equipments in the ports so as to be used as a model for automating processes of identifying, shipping, and loading each item.
Currently, in order to identify and trace items in ports, workers carry terminals such as personal digital assistants (PDAs), identify the existence and state of the items by hand, and transmit information to a server through a wireless local area network (LAN) installed in the ports. If the workers are skilled, accurate information may be collected and maintained through the above processes. However, personnel expenses may increase and the state of the items that are changed every hour may not be rapidly identified.
In this regard, a RFID technology that has currently been highlighted may solve such problems above. That is, if RFID tags are attached to each container and a collection command is sent to the RFID tags by using a reader, information of each RFID tag may be automatically collected through wireless radio. Thus, it is possible to rapidly cope with unexpected circumstances where items are damaged or stolen without checking one by one by the workers.
FIG. 1 is a view for describing an operation of a current RFID standard system.
Referring to FIG. 1, a command of a reader 100 is transmitted to surrounding tags by wireless and the tags transmit response messages including their own information to the reader 100. Then, the reader 100 provides the response messages received from the tags to a manager terminal 500 through a server 400 so that a manager may identify the existence and state of items at a long distance. However, a fifth tag 200, which cannot receive a signal from the reader 100, may exist due to an obstacle (wall) 300. In this case, the manager may have difficulties in identifying the existence and state of items.
For example, if the wall 300, which interrupts a radio wave flowing between the reader 100 and the fifth tag 200, exists, the fifth tag 200 may not receive a signal from the reader 100 and thus may not respond to the reader 100. Then, the reader 100 may not recognize the existence of the fifth tag 200.
An area where a signal of the reader 100 does not reach is called as a shadow area. In container ports, metal containers are loaded in four columns and five rows and large-sized equipments such as cranes and yard tractors are scattered so that there is a strong probability that the shadows are generated due to an interruption of radio wave by the containers or the large-sized equipments. In particular, the recognition distance of the active tags having a frequency of 433 MHz used in port logistics is known as 50-100 m. However, in special environments such as ports, the recognition distance is actually less than 50-100 m and vanes according to the loaded form of the items.
As such, if the information of all tags is not completely recognized in the reader 100, serious problems may occur and affect the efficiency of the entire port logistics. That is, containers in which their types, destinations and existence are not accurately recognized may adversely affect the stored form, shipping, and loading scheduling of the entire containers. This is because the current international standard (ISO/IEC 18000-7) for communication using active tags having a frequency of 433 MHz for port logistics is defined only for direct communication between a reader and a tag and is not appropriate to support multi-hop communication between a reader and a tag.
Accordingly, tag information that is not recognized due to the communication shadows or instability of wireless communication needs to be recorded again by hand. Thus, the current RFID standard system does not have strong advantages compared with an environment where an existing wireless LAN is used. Accordingly, a method for solving the problem of the communication shadows is needed in the RFID technology so as to contribute to automation of the port logistics.
Current technologies for solving the problem of the communication shadows are as follows.
Firstly, a fixed-type wireless reader may be further installed in a place where the shadow is frequently generated. However, there are a number of problems in this method. One of the main problems is that the place, to which the fixed-type reader can be installed in a port, is limited. Since moving paths for yard tractors or forklifts may be secured between densely stored containers and spaces for equipments such as cranes or lifts are needed, the place, to which the reader can be installed, is limited to lighting towers located in a container storage yard. In addition, even if the fixed-type reader is installed, smooth communication between the reader and the tag may be obstructed due to metal-formed large-sized equipments and containers.
Secondly, mobile readers may be developed and introduced. The mobile readers may be attached to working vehicles in a port and the working vehicles continuously supply a power source to the mobile readers. Thus, the mobile readers collect information of tags. However, there are some problems in the mobile readers. For example, communication shadow area may be continuously generated in specific zones. In inaccessible zones, for example, most distant zones from the moving paths of the working vehicles or tags attached to containers stored at the highest place, the mobile readers may not communicate. Also, the mobile readers continuously move so that a message load is generated due to frequent changes of network topology, and stability of a network is hardly secured. In addition, a price of the mobile reader is higher by 2-3 times than that of the fixed-type reader and thus the price is another problem of the mobile readers.
Accordingly, the current RFID system in the port logistics may not solve the problems of the shadow areas that are irregularly generated. In this regard, the readers may be installed more densely; however, expenses may increase in this case and signals of the readers may collide with each other. Moreover, spaces for storing containers and moving/operating equipments have to be secured and thus spaces for installing the readers are limited.